770G and 772G Off-Highway Truck (OEM) Hydraulic and Braking System: Control Valve (Hoist)

(1) Hoist proportioning solenoid for SNUB, FLOAT, and LOWER (2) High pressure relief valve (3) Low pressure relief valve (4) Hoist proportioning solenoid for RAISE (5) Brake oil cooler relief valve (6) Outlet to the hydraulic tank (7) Outlet to the head end of the hoist cylinders (8) Outlet to the rod end of the hoist cylinders (9) Outlet to the brake oil cooler(10) Inlet for pump oil

(1) Hoist proportioning solenoid for SNUB, FLOAT, and LOWER (2) High pressure relief valve (3) Low pressure relief valve (4) Hoist proportioning solenoid for RAISE (5) Brake oil cooler relief valve (6) Outlet to the hydraulic tank (7) Outlet to the head end of the hoist cylinders (8) Outlet to the rod end of the hoist cylinders (9) Outlet line to the brake oil cooler (10) Inlet line for pump oil (11) Pilot oil line from the brake retract system (12) Actuators (13) Hoist control valve (14) Centering springs (15) Dump spool (16) Stem (17) Directional spool (18) Orifices (19) Hoist cylinders (20) Pump drive (21) Hoist pump(22) Hydraulic tank

Part of the pump oil from the parking brake system goes to hoist control valve (13) through line (11). This oil is used as pilot oil for hoist proportioning solenoid (1) and hoist proportioning solenoid (4). Pilot oil is always present at both ends of directional spool (17). The hoist proportioning solenoids are used to drain the pilot oil from the ends of directional spool (17), which then allows the spool to move.

Oil from hoist pump (21) flows through line (10) to hoist control valve (13). The hoist control valve will send pump oil to one of two locations. When the dump body is being raised or lowered, hoist cylinders (19) receive the oil. If the hoist is not being used, the oil flows through line (9) to the brake oil cooler.

Brake oil cooler relief valve (5) limits the pressure in line (9) to the brake oil cooler. Brake oil cooler relief valve (5) for the brake oil cooler will vent oil to the hydraulic tank (22), when the pressure in line (9) is too high.

Low pressure relief valve (3) limits the pressure in line (8) to the rod end of hoist cylinders (19) when the dump body is being lowered. Low pressure relief valve (3) will open when the pressure in line (8) becomes too high. When low pressure relief valve (3) opens, dump spool (15) will open and oil will vent to the hydraulic tank (22) .

When the dump body is being raised oil is sent to stem (16). Stem (16) will shift. Access to the low pressure relief valve (3) will be blocked. The high pressure relief valve (2) will now limit the pressure in line (7) to the head end of hoist cylinders (19). High pressure relief valve (2) will open when the pressure in line (7) becomes too high. When high pressure relief valve (2) opens, dump spool (15) will open and oil will vent to the hydraulic tank (22) .

The electric tow pump can also be used to send pilot oil to the hoist control valve (13). If the body needs to be lowered with a dead engine, the electric tow pump will provide the pilot oil through the diverter valve. The tow pump will only send pilot oil to the hoist control valve if the diverter valve is manually opened.

HOLD Position

In the HOLD position, neither of the hoist proportioning solenoids are receiving current. All of the pilot oil is blocked in actuators (12). Centering springs (14) hold directional spool (17) so that line (7) to the head end of hoist cylinders (19) is blocked by directional spool (17). Line (8) to the rod end of hoist cylinders (19) is also blocked by directional spool (17) .

The directional spool (17) is in a position that allows all of the pump oil to flow to the brake cooling system through line (9). The maximum pressure in line (9) to the brake cooling system is controlled by brake oil cooler relief valve (5). If brake oil cooler relief valve (5) opens, the oil will vent through line (6) to the hydraulic tank (22) .

RAISE Position

In the RAISE position, hoist proportioning solenoid (4) is energized and the pilot oil in actuator (12) for hoist proportioning solenoid (4) is allowed to drain to the hydraulic tank (22). Pilot oil in actuator (12) for hoist proportioning solenoid (1) will now push directional spool (17) in an upward direction. (Refer to the particular orientation of control valve (13) in Illustration 2.) Once directional spool (17) moves, the pump oil that is coming through inlet (10) flows to line (7), and to the head end of hoist cylinders (19). The position of directional spool (17) allows oil from the rod end of hoist cylinders (19) to flow to the brake oil cooler through line (9). Orifices (18) control the rate of oil that is leaving the rod end of hoist cylinders (19). This will prevent hoist cylinders (19) from moving too fast for hoist pump (21) to supply oil. This can happen when a load shifts during the dump.

Some of the oil in line (7) is sent as signal oil to stem (16). The signal pressure shifts stem (16) so that low pressure relief valve (3) is blocked. This allows all of the pump oil that is flowing through dump spool (15) to flow to high pressure relief valve (2). If the oil pressure opens high pressure relief valve (2), dump spool (15) will shift to the right. (Refer to the particular orientation of control valve (13) in Illustration 2.) The pump oil will then be vented through line (6) to the hydraulic tank (22) .

LOWER Position

The LOWER position is also referred to as the POWER DOWN position. In the LOWER position, hoist proportioning solenoid (1) is energized with a high current. Pilot oil in actuator (12) for hoist proportioning solenoid (1) is allowed to drain to the hydraulic tank (22). Pilot oil in actuator (12) for hoist proportioning solenoid (4) will now push directional spool (17) in a downward direction. (Refer to the particular orientation of control valve (13) in Illustration 2.) Once directional spool (17) moves, the pump oil that is coming through inlet (10) flows through line (8) to the rod end of hoist cylinders (19). Also, oil from the head end of hoist cylinders (19) is allowed to drain to the hydraulic tank (22) .

The rate of descent of the truck body is controlled by the flow rate of oil that drains from the head end of hoist cylinders (19). When the directional spool moves down, this oil drains through openings at the lower end of directional spool (17) into line (6). (Refer to the particular orientation of control valve (13) in Illustration 2.) The size of the drain opening that is exposed to line (6) varies with the degree of travel of the directional spool. Greater travel of directional spool (17) will expose more of the drain passage. Greater travel of directional spool (17) will therefore allow the oil in the head end of hoist cylinders (19) to drain more rapidly. When oil drains quickly from the head end of the hoist cylinders, the truck body descends quickly.

In the LOWER position, a high current is used to energize hoist proportioning solenoid (1). Pilot oil in accumulator (12) for proportioning solenoid (1) drains at a rapid rate. Consequently, the directional spool moves downward far enough to create a large drain opening. The truck body will descend rapidly.

Because no signal pressure is sent to stem (16), pump oil can flow to low pressure relief valve (2). If low pressure relief valve (2) opens, dump spool (15) will shift to the right. (Refer to the particular orientation of control valve (13) in Illustration 2.) The pump oil will then be vented through line (6) to the hydraulic tank (22) .

FLOAT Position

In the FLOAT position, hoist proportioning solenoid (1) is energized with a moderate current. Pilot oil in actuator (12) for hoist proportioning solenoid (1) is allowed to drain to the hydraulic tank (22). Pilot oil in actuator (12) for the hoist proportioning solenoid (4) will now push directional spool (17) in a downward direction. (Refer to the particular orientation of control valve (13) in Illustration 2.) Once the directional spool (17) moves, some of the pump oil that is coming through inlet (10) flows to the rod end of hoist cylinders (19). The remaining pump oil is sent to the rear brake cooling system through line (9). Also, oil from the head end of hoist cylinders (19) is allowed to drain to the hydraulic tank (22) .

The rate of descent of the truck body is controlled by the flow rate of oil that drains from the head end of hoist cylinders (19). When the directional spool moves down, this oil drains through openings at the lower end of directional spool (17) into line (6). (Refer to the particular orientation of control valve (13) in Illustration 2.) The size of the drain opening that is exposed to line (6) varies with the degree of travel of the directional spool. Shorter travel of directional spool (17) will expose less of the drain passage. Shorter travel of directional spool (17) will therefore allow the oil in the head end of hoist cylinders (19) to drain more slowly. When oil drains more slowly from the head end of the hoist cylinders, the truck body descends more slowly.

In the FLOAT position, a moderate current is used to energize hoist proportioning solenoid (1). Pilot oil in actuator (12) for proportioning solenoid (1) drains at a moderate rate. Consequently, the directional spool moves downward far enough to create a moderately sized drain opening. The truck body will descend at a moderate rate.

Because no signal pressure is sent to stem (16), pump oil can flow to low pressure relief valve (2). If low pressure relief valve (2) opens, dump spool (15) will shift to the right. (Refer to the particular orientation of control valve (13) in Illustration 2.) The pump oil will then be vented through line (6) to the hydraulic tank (22) .

SNUB Position

The SNUB position is activated by the Power Train Electronic Control Module (ECM). The operator has no control over this position. The hoist control valve will be in the SNUB position when both of the following conditions are met: the hoist lever is in the LOWER position or in the FLOAT position and the body down or up switch is in the DOWN position.

The SNUB position will slow the descent of the dump body as the dump body nears the frame. A signal from the body down or up switch will be sent to the Power Train ECM when the dump body is near the frame. The Power Train ECM will then send a signal in order to move the hoist control valve to the SNUB position.

In the SNUB position, hoist proportioning solenoid (1) is energized with a low current. Pilot oil in actuator (12) for hoist proportioning solenoid (1) is allowed to drain to the hydraulic tank (22). Pilot oil in the actuator (12) for the hoist proportioning solenoid (4) will move directional spool (17) downward. (Refer to the particular orientation of control valve (13) in Illustration 2.) In the SNUB position, some of the pump oil that is coming through inlet (10) flows to the rod end of the hoist cylinders (19). The remaining pump oil from line (10) is sent to the brake cooling system through line (9). Also, oil from the head end of hoist cylinders (19) is allowed to drain to the hydraulic tank (22) .

The rate of descent of the truck body is controlled by the flow rate of oil that drains from the head end of hoist cylinders (19). When the directional spool moves down, this oil drains through openings at the lower end of directional spool (17) into line (6). (Refer to the particular orientation of control valve (13) in Illustration 2.) The size of the drain opening that is exposed to line (6) varies with the degree of travel of the directional spool. Shorter travel of directional spool (17) will expose less of the drain passage. Shorter travel of directional spool (17) will therefore allow the oil in the head end of hoist cylinders (19) to drain more slowly. When oil drains more slowly from the head end of the hoist cylinders, the truck body descends more slowly.

In the SNUB position, a minimal amperage is used to energize hoist proportioning solenoid (1). Pilot oil in actuator (12) for proportioning solenoid (1) drains at a slow rate. Consequently, the directional spool moves just far enough to create a small sized drain opening. The truck body will descend slowly to the frame.

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770G and 772G Off-Highway Truck (OEM) Hydraulic and Braking System: Control Valve (Hoist) 2013-01-04T14:31:00-08:00 Rating: 4.5 Diposkan Oleh: Unknown